This invention relates to a footwear sole structure, particularly for mountaineering or rock climbing boots.
Such soles are at present generally made from rubber by a hot molding process.
A basic common feature of such sole structures is to be found in the pattern conferred to the lower or bottom side of said soles, i.e., on that part which comes in direct contact with the ground. That pattern must be such as to develop adequate adhesion forces, especially along exceptionally steep ground or rock.
To this aim, the lower side of said soles is provided with ribs or lugs which, in most instances, i.e., both on the sole proper and the heel, extend substantially perpendicular to the sole longitudinal direction and, in all instances, project perpendicularly from the sole bottom.
That constructional arrangement, i.e., where the ribs or lugs project perpendicularly from the sole, although currently adopted for the soles of all the mountaineering boots, seems incapable of ensuring adhesive power and anti-slip properties at their best, thereby it is not entirely devoid of shortcomings.
Indeed, if such lugs project perpendicularly from the sole, they also stand perpendicular to the ground whereon they are resting. Therefore, between the vertical walls of such ribs or lugs and the ground irregularities, which by wedging themselves in between such lugs come in contact and engagement with those vertical walls of said lugs -- thus creating that frictional force or reaction which should prevent the sole from slipping -- a sharp corner is formed; if the force exerted by the foot in a longitudinal direction becomes, even though only momentarily, greater than the frictional reaction, the sole slips, since between its lugs and the ground irregularities there is lacking that mutual engagement which created the frictional reaction.
Thus, to sum it up, the frictional reaction exerted by the sole against the ground is such as to overcome the force applied to the sole by the foot up to a given value, but once that value is exceeded, the frictional reaction becomes practically nil, thereby the wearer of the boots including such soles is caught unaware and may find himself in some danger.